The most basic way to adapt sound to the needs of hearing impaired individuals is to simply amplify the sound. However, many times amplification is not sufficient, for example, if the hearing loss for a particular frequency is to large such that the maximum output level of the device is reached before the sound can be perceived by the individual. Sometimes there are so called “dead regions”, which means that sounds of specific frequencies cannot be perceived at all no matter how much they are amplified. In view of this, devices have been developed which do not simply amplify, but also change the frequency of spectral components such that they can be perceived in frequency regions where the hearing of the individual is better.
U.S. Pat. No. 5,014,319 discloses a frequency transposing hearing aid. The hearing aid apparatus comprises a pair of analogue delay lines. A transposition factor is a ratio of information storage rate to information retrieval rate. There are means for inputting at least two different transposition coefficients predetermined according to the user's hearing characteristics for different frequencies. There are frequency analyzer means to select the appropriate transposition coefficient according to the frequency of the incoming signal.
U.S. Pat. No. 5,394,475 discloses a device for transposing the frequency of an input signal. It may be provided that a momentary frequency signal is subjected to a controlling means. In this way it is possible to change the extent of frequency shift. The control can be made manually through a potentiometer by the carrier of the hearing aid or depending on the volume encountered. A non-linear transformer can be provided to shift individual frequency ranges to different extents. The document mentions digital technology and Fourier transformation.
U.S. Pat. No. 6,577,739 discloses an apparatus for proportional audio compression and frequency shifting. The fast Fourier transform of the input signal is generated, to allow processing in the frequency domain. By proportionally shifting the spectral components the lawful relationship between spectral peaks associated with speech signals is maintained so the listener can understand the information.
AU 2002/300314 discloses a method for frequency transposition in hearing aids. Preferably, a fast Fourier transform is used. In an example input frequencies up to 1000 Hz are conveyed to the output of the hearing-aid without any shifting. Frequencies above 1000 Hz are shifted downwards progressively such that an input frequency of 4000 Hz is conveyed to the output after being transposed downwards by one octave, to produce an output frequency of 2000 Hz.
U.S. Pat. No. 7,248,711 discloses a method for frequency transposition in a hearing device. There is a nonlinear frequency transposition function. Thereby, it is possible to transpose lower frequencies almost linearly, while higher frequencies are transposed more strongly. As a result thereof, harmonic relationships are not distorted in the lower frequency range. In an embodiment the frequency transposition function has a perception based scale. In regard to frequency compression fitting it is mentioned that there are the parameters compression ratio above the cut-off frequency and cut-off frequency.
WO 2007/000161 discloses a hearing aid for reproducing frequencies above the upper frequency limit of a hearing impaired user. There are means for transposing higher bands down in frequency. There are means for superimposing the transposed signal onto an other signal creating a sum signal. The transposition down in frequency can be by a fixed amount, e.g. an octave.
DE 10 2006 019 728 discloses a time-adaptive hearing aid device. A part of the input spectrum is shifted automatically from a first frequency to a second frequency as a function of time. Thereby a time-adaptive parameterisation of the compression ratio is achieved. The spontaneous acceptance of a hearing system is improved and there is support for the acclimatization of the hearing impaired to new frequency patterns.
Generally it can be concluded that there are numerous frequency modification schemes known in the state of the art. However, each of them is somehow imperfect in regard to one or more of the following aspects:                Finding an optimum trade-off between the presence of artefacts, disturbing noises or disharmonies and an improved intelligibility of speech;        Allowing a reasonable technical implementation, which includes issues such as circuit complexity, power consumption and processor load;        Avoiding information loss which may be caused by superposition of signals or incomplete playback when signals are played back at a reduced speed;        Opening up the possibility to provide solutions for individuals with mild or moderate hearing losses.        